The present invention relates to a method of preparing polyester and, more particularly, such polyester of which at least 85 mol % of the recurring unit comprises alkylene aromatic dicarboxylate.
In conventional methods for the preparation of aromatic polyesters, especially polyalkyleneterephthalates, terephthalic acid or dimethyl terephthalate and alkylene glycol such as ethylene glycol are subjected to esterification or interesterification in the absence or presence of a catalyst to produce bis-(2-hydroxyethyl)terephthalate or its oligomers, and the polyester intermediate or precursor, which is mainly composed of bis-(2-hydroxyethyl)terephthalate and/or its oligomers obtained above, is continuously charged into a polycondensation reaction vessel to be polycondensated. Such a conventional method is shown in for example the specification of U.S. Pat. No. 3,499,873.
The above conventional method is explained in detail along with FIG. 1 as follows.
The polycondensation reaction vessel in FIG. 1 is provided with a cylindrical casing 2 having therein a stirring means 1 and being heated through a jacket 7, an inlet 3 for introducing the polyester intermediate or precursor from a terminal portion of the body, an outlet 4 for taking the obtained reactant out of another terminal portion of the casing and an opening 5 leading to a source of vacuum, via which alkylene glycol is with drawn. Liquid phase L and gas phase S mean the polycondensation reaction mixture and another part in casing 2, respectively.
When the polyester intermediate or precursor is subjected to polycondation under a high vacuum condition at an elevated temperature, normally there be come unreacted monomers and oligomers separated as entrained by alkylene glycol evaporating from the reaction mixture (such monomers and oligomers will be referred to as sublimable substances or sublimates in the following description). Therefore, if the above polycondation is operated for a long period of time, likely is that a portion of the sublimable substance becomes attached onto and accumulated on the surface of an inner wall 6 surrounding the gas phase S in the casing 2. Further, when subjected to the reduced pressure condition for a long period of time, deposited sublimates undergo, through heat experience, gelling or the like to become thermally denatured matters and, falling into the reaction mixture, become mixed with the same eventually.
Such thermally denatured substance, which increases in its amount as the time lapses, functions to adversely affect the quality of polyester to be produced and cause to occur various troubles at the time of spinning, drawing, molding or other anticipated steps.
Accordingly, one of the best ways of obtaining a desirable and constant quality of polyester fiber or a molded polyester product is to prevent the deposition of sublimates onto the inner wall of the polycondensation reaction vessel.
However, such a method has not yet been established, so that it is forced in the art today to employ a method of intermittently washing the inner wall, but this method necessarily accompanies a lowering of the rate of operation in the continuous production of polyester using the above mentioned apparatus. Therefore, a device free of the need for periodically washing the inner wall is strongly desired.